Fig. 2666.
When a belt is under tension upon two pulleys and at rest, the friction or grip of the belt upon the respective pulleys (supposing them to be of the same diameter and therefore to have the same arc and area of contact) will depend upon the relative positions of the pulleys; thus suppose one pulley to be above the other as in [Fig. 2665], the upper pulley p will have the grip due to the tension of the belt added to that due to the weight of the belt, whereas if placed horizontally, as in [Fig. 2666], the weight of the belt will fall equally on the two pulleys, and for this reason vertical belts of a given width require to have a greater tension to transmit the same amount of power as the same belt would if placed horizontally. But as soon as motion was transmitted, by the belt, from one pulley to the other, the belt on one side of the pulley would be under greater tension then that on the other.
Fig. 2667.
Suppose, for example, a belt to transmit motion and power from pulley a in [Fig. 2667], to pulley b, then the side c of the belt is that which drives or pulls b, and it is therefore called the driving side of the belt, the resistance to rotation offered by b causing the driving side of the belt to be the most strained; and hence the straightest, whereas the side d will be free of the tension due to the resistance of b.
